It is well known that ultraviolet radiation causes colorants to degrade. This is an acute problem for articles exposed to solar radiation for extended periods of time, such as traffic signs. Color degradation occurs in articles colored with conventional colorants as well as in articles colored with fluorescent colorants, which tend to degrade more quickly than conventional dyes. Conventional colorants are those that do not exhibit fluorescent properties to the unaided eye. Whereas the life of materials with fluorescent colorants exposed to daily solar radiation is in the range of months, the life of materials with conventional colorants is typically in the range of years.
If increased visibility or conspicuity of an article is desired, it is often colored with fluorescent colorants even though they are less durable than conventional colorants. Fluorescent colors provide enhanced visibility because the visual contrast that the fluorescent colors create with the environment make the materials more conspicuous than non-fluorescent articles. Fluorescent colored traffic signs are effective at increasing the visibility of the signs thereby increasing motorists awareness to the surrounding possibly leading to increased safety. Even though fluorescent colored signs help to increase motorist safety, their use has been limited due to inadequate color stability.
Attempts have been made to try to maintain the fluorescent colors in retroreflective sheeting. For example, Japanese application JP-A-6-43189 discloses a multilayered retroreflective sheeting comprising a retroreflective base sheet having a retroreflective side, a first transparent colored layer containing a transparent pigment laminated to the retroreflective side of the base sheet, and a second transparent colored layer containing a fluorescent dye laminated to the first transparent colored layer. The reference further discloses that when the transparent pigment and the fluorescent dye are contained in the same layer, the pigment interferes with the fluorescent dye. Thus, there is a need to put the pigment and fluorescent dyes in separate layers.
Japanese application JP-A-6-160615 discloses a fluorescent retroreflective sheet comprising a transparent protective film, a transparent binder layer having glass beads with an associated reflector (such as aluminum vapor coat) partially embedded therein, and a layer containing fluorescent coloring agents disposed behind the binder layer. However, the sheeting has poor fluorescent properties because the fluorescent material is limited to areas where there are no glass beads.
U.S. Pat. No. 5,387,458 (Pavelka et al.) discloses a fluorescent retroreflective sheet comprising a transparent overlay containing a hindered amine light stabilizer, a polymeric layer containing a fluorescent dye, and optionally a cube corner layer. Although the sheeting has effective, durable, fluorescent properties and is therefore very useful in many traffic related applications such as traffic signs, it may not be as useful in applications where it is desirable to highlight only selected portions of a traffic sign. For example, if it is desirable to have symbols on a traffic sign in fluorescent color, one may need to cut the symbols from this sheeting and laminate them to a sign.
A need still exists for retroreflective sheetings that exhibit good retroreflectivity and good fluorescent properties in selected portions of the sheeting without the cumbersome process of cutting the desired symbols and laminating them to a substrate.